Smart cast to 'Type' is impossible, because 'variable' is a mutable property that could have been changed by this time

Question

And the Kotlin newbie asks, "why won't the following code compile?":

var left: Node? = null
    
fun show() {
    if (left != null) {
        queue.add(left) // ERROR HERE
    }
}

Smart cast to 'Node' is impossible, because 'left' is a mutable property that could have been changed by this time

I get that left is mutable variable, but I'm explicitly checking left != null and left is of type Node so why can't it be smart-casted to that type?

How can I fix this elegantly?

Answer 1

Between execution of left != null and queue.add(left) another thread could have changed the value of left to null.

To work around this you have several options. Here are some:

1. Use a local variable with smart cast:

    val node = left
    if (node != null) {
        queue.add(node)
    }
   

2. Use a safe call such as one of the following:

    left?.let { node -> queue.add(node) }
    left?.let { queue.add(it) }
    left?.let(queue::add)
   

3. Use the Elvis operator with return to return early from the enclosing function:

    queue.add(left ?: return)
   

   Note that break and continue can be used similarly for checks within loops.

Answer 2

1) Also you can use lateinit If you sure do your initialization later on onCreate() or elsewhere.

Use this

lateinit var left: Node

Instead of this

var left: Node? = null

2) And there is other way that use !! end of variable when you use it like this

queue.add(left!!) // add !!

Answer 3

There is a fourth option in addition to the ones in mfulton26's answer.

By using the ?. operator it is possible to call methods as well as fields without dealing with let or using local variables.

Some code for context:

var factory: ServerSocketFactory = SSLServerSocketFactory.getDefault();
socket = factory.createServerSocket(port)
socket.close()//smartcast impossible
socket?.close()//Smartcast possible. And works when called

It works with methods, fields and all the other things I tried to get it to work.

So in order to solve the issue, instead of having to use manual casts or using local variables, you can use ?. to call the methods.

For reference, this was tested in Kotlin 1.1.4-3, but also tested in 1.1.51 and 1.1.60. There's no guarantee it works on other versions, it could be a new feature.

Using the ?. operator can't be used in your case since it's a passed variable that's the problem. The Elvis operator can be used as an alternative, and it's probably the one that requires the least amount of code. Instead of using continue though, return could also be used.

Using manual casting could also be an option, but this isn't null safe:

queue.add(left as Node);

Meaning if left has changed on a different thread, the program will crash.

Answer 4

Change var left: Node? = null to lateinit var left: Node. Problem solved.

Answer 5

The practical reason why this doesn't work is not related to threads. The point is that node.left is effectively translated into node.getLeft().

This property getter might be defined as:

val left get() = if (Math.random() < 0.5) null else leftPtr

Therefore two calls might not return the same result.

Answer 6

Your most elegant solution must be:

var left: Node? = null

fun show() {
    left?.also {
        queue.add( it )
    }
}

Then you don't have to define a new and unnecessary local variable, and you don't have any new assertions or casts (which are not DRY). Other scope functions could also work so choose your favourite.

Answer 7

Do this:

var left: Node? = null

fun show() {
     val left = left
     if (left != null) {
         queue.add(left) // safe cast succeeds
     }
}

Which seems to be the first option provided by the accepted answer, but that's what you're looking for.

Answer 8

For there to be a Smart Cast of the properties, the data type of the property must be the class that contains the method or behavior that you want to access and NOT that the property is of the type of the super class.

---

e.g on Android

Be:

class MyVM : ViewModel() {
    fun onClick() {}
}

Solution:

From: private lateinit var viewModel: ViewModel
To: private lateinit var viewModel: MyVM

Usage:

viewModel = ViewModelProvider(this)[MyVM::class.java]
viewModel.onClick {}

GL

Answer 9

Try using the not-null assertion operator...

queue.add(left!!) 

Answer 10

How I would write it:

var left: Node? = null

fun show() {
     val left = left ?: return
     queue.add(left) // no error because we return if it is null
}